1. Field of the Invention
The present invention relates to a fluoroelastomer and its cross-linkable composition and more particularly to a fluoroelastomer capable of producing vulcanization products having distinguished low-temperature characteristics and methanol resistance, and its vulcanizable composition.
2. Related Prior Art
(1) An elastomeric terpolymer comprising 57 to 61% by weight (71.1 to 76.0% by mole) of vinylidene fluoride, 10 to 14% by weight (13.5 to 17.9% by mole) of tetrafluoroethylene and 27 to 31% by weight (8.1 to 11.6% by mole) of hexafluoropropene, the sum total being 100% by weight (or by mole), disclosed in JP-B-62-1965 (U.S. Pat. No. 4,123,603), produces vulcanization products having a low glass transition temperature (Tg), a low compression set at low temperature and low pressure, and also a good methanol resistance, but have no satisfactory level of low-temperature characteristics (TR-10) yet. The elastomeric terpolymer has a low vulcanization speed with a polyol-based cross-linking agent. Thus, injection molding, etc. cannot be effectively used for the elastomeric terpolymer.
(2) A fluoroelastomer comprising 60.5 to 64% by weight (76.6 to 79.4% by mole) of vinylidene fluoride, 5 to 8% by weight (4.0 to 6.5% by mole) of tetrafluoroethylene and 30 to 33% by weight (15.0 to 17.4% by mole) of hexafluoropropene, the sum total being 100% by weight (or by mole), disclosed in JP-A-6-49138 (Italian Patent Application No. 41,003A/90), discloses its vulcanization products having not only the similar low-temperature characteristics to those of the vulcanization products from the elastomeric terpolymer (1), but also a lower TR (temperature contraction) value and a lower degree of crystallization and furthermore a higher vulcanization speed than that of the vulcanization products from the elastomeric terpolymer (1). But, as the vulcanization products from the fluoroelastomer (2) has a poor methanol resistance, its applications are limited. To overcome the poor methanol resistance by adjusting the vulcanizable composition with some additives, i.e. a filler, a mixing ratio of the filler must be increased, resulting in an increase in the hardness of vulcanization products. Its applications are likewise limited.
As to the low-temperature characteristics to be good, TR-70 is still below the satisfactory level. To improve TR-70 within the said composition range of the fluoroelastomer, it would be effective to increase the proportion of hexafluoropropene, but the increase of hexafluoropropene would inevitably deteriorate a TR-10 value of vulcanization products. That is, it is difficult to maintain a good balance between physical properties. As to the hardness of vulcanization products, the hardness tends to unfavorably increase with time in the said composition range of the fluoroelastomer.
(3) A fluoroelastomeric copolymer comprising 48 to 65% by weight (64.7 to 81.4% by mole) of vinylidene fluoride, 0 to 17% by weight (0 to 15% by mole) of tetrafluoroethylene, 21 to 36% by weight (10.8 to 21.9% by mole) of hexafluoropropene and 3 to 9% by weight (1.4 to 4.9% by mole) of perfluoro(alkyl vinyl ether), the sum total being 100% by weight (or by mole), where "% by mole" shows proportions of individual comonomers, when perfluoro(methyl vinyl ether) is used as perfluoro(alkyl vinyl ether), disclosed in JP-A-6-128334, is directed to improvement of the low-temperature characteristics of vulcanization products from the said fluoroelastomer (2), where a portion of hexafluoropropene of the fluoroelastomer (2) is replaced with about 8% by weight (about 4% by mole) of perfluoro(methyl vinyl ether) in the said composition range of the fluoroelastomeric copolymer (2), whereby the TR-10 value of the vulcanization products was inproved by 3.degree. C., as shown in Tables 3 and 4.
However, the fluoroelastomeric copolymer within the above-mentioned composition range has a low vulcanization speed with a polyol-based cross-linking agent. To obtain a practical vulcanization speed, the mixing ratio of a vulcanization promoter must be increased, resulting in an increase in the vulcanization minimum torque. Injection molding thus will be unfavorable.
(4) A fluoroelastomeric copolymer comprising 30 to 47% by weight (45.4 to 67.5% by mole) of vinylidene fluoride, 10 to 30% by weight (8.9 to 29.4% by mole) of tetrafluoroethylene, 18 to 40% by weight (11.2 to 27.9% by mole) of hexafluoropropene and 3 to 20% by weight (1.5 to 11.7% by mole) of perfluoro(alkyl vinyl ether), the sum total being 100% by weight (or by mole), where "% by mole" shows proportions of individual comonomers, when perfluoro(methyl vinyl ether) is used as perfluoro(alkyl vinyl ether), disclosed in JP-A-6-145252, is suitable for the production of shaft seals and fuel hoses in contact with motor oil or gasoline containing an alcohol, particularly methanol, but its vulcanization products have low-temperature characteristics, for example a TR-10 value as low as -13 to -16.5.degree. C. Thus, the service temperature will be limited.
(5) A fluoroelastomer comprising 48 to 65% by weight (66. 1 to 87.0% by mole) of vinylidene fluoride, 4 to 15% by weight (2.2 to 13.3% by mole) of tetrafluoroethylene, 8 to 23% by weight (2.9 to 13.9% by mole) of hexafluoropropene and 17 to 30% by weight (5.6 to 16.1% by mole) of perfluoro(alkyl vinyl ether), the sum total being 100% by weight (or by mole), where "% by mole" shows proportions of individual comonomers, when perfluoro(methyl vinyl ether) is used as perfluoro(alkyl vinyl ether), disclosed in JP-B-53-4035, shows that the vulcanization products have good low-temperature characteristics, but its Examples show the fluoroelastomer has a low vulcanization speed, that is, requirements of 168.degree. C. for 20 minutes (Example 5) or 177.degree. C. for 15 minutes (Example 7) for the primary vulcanization and further requirements of long time for the post-curing. Thus, injection molding will be difficult to conduct.